This allows an application to register a custom EDNS option type.

ECSOption and GenericOption both contained a __str__ method that called
to_text(), but the base Option class did not.  Move __str__ to the base
class.

Wire format data is now done via a dns.wire.Parser, which does all of the
bookkeeping and also provides convenience routines (e.g. get_uint16() or
get_name()).

of two objects of the same class.

Previously we did type-specific stuff for ECSOption, but we only
checked the padded address info, not the srclen or scopelen, so
different options could compare the same.  Also, it's not clear that
there's any better semantic ordering of options than the wire format,
so we will just go for simplicity and use the generic implementation.